It is well-known that the sensitivity of a frequency modulated radio receiver is significantly degraded on channels where the received frequency is equal to a harmonic frequency of the receiver clock frequency. This problem, known as co-channel interference, is a form of "jamming", and can be understood by reference to FIG. 1. FIG. 1 is a block diagram of a prior art single stage RF receiver that can exhibit the problem solved by the present invention. While a single stage RF receiver is shown, the problem of co-channel interference arises with receivers having multiple stages or direct-to-digital down conversion.
In the example shown, a modulated RF signal 100 having a particular carrier frequency, F.sub.RFC =N.multidot.F.sub.CH (where N is an integer designating, in effect, a channel), is generated in known fashion by a remote RF transmitter and received at an antenna 102 of a compatible receiver. When the audio content of the RF signal 100 is zero (that is, when an audio signal is not present in the received signal). F.sub.RFC is constant. Otherwise, F.sub.RFC is modulated in a manner which represents the audio content of the transmitted and received RF signal 100. The instantaneous modulated frequency is F.sub.RFC +F.sub.MOD.
The received signal is typically amplified by a low noise amplifier 104 before conversion by a conventional down converter 106 to a baseband frequency (for a single stage down converter) or to an intermediate frequency (which is further down converted by one or more subsequent stages until a baseband frequency is generated).
The other input to the down converter 106 is an "injection" frequency, which is synthesized to have a nominal frequency of N.multidot.F.sub.CH (where N is an integer designating, in effect, a channel). In the illustrated embodiment, a processor 108 (e.g., a digital signal processor, or DSP) coupled to a clock source 109 (e.g., a crystal) outputs a control voltage having a nominal value of Vo to an oscillator 110, which is preferably a voltage controlled crystal oscillator (VCXO) but may be a voltage controlled oscillator (VCO). Typically, the oscillator 110 outputs a reference frequency F.sub.REF, which equals k.multidot.Vo, where k is a constant. By way of example only, for certain cellular telephone systems (e.g., GSM), F.sub.REF may be 13 MHz or 19.5 MHz. This reference frequency is then divided down in a divider 111 by a value R to a channel spacing frequency, F.sub.Ch. The channel spacing frequency F.sub.CH s applied to a multiplier circuit 112 (e.g., a phase-locked loop), which outputs the injection frequency of N.multidot.FCH for a particular "channel" mapped to a multiplier value N.
The down converter 106 subtracts the injection frequency N.multidot.F.sub.CH from the received frequency F.sub.RFC +F.sub.MOD (which equals N.multidot.F.sub.CH +F.sub.MOD) to generate a baseband frequency F.sub.MOD representing the audio content of the received signal 100. In the illustrated embodiment, the baseband frequency F.sub.MOD is converted to a digital frequency-modulated (FM) waveform by an analog-to-digital converter 114. The processor 108, which generally includes a memory circuit 116 for data and program storage, decodes the FM signal and outputs a digital audio signal. A digital-to-analog converter 118 converts the digital audio signal to an analog audio waveform, which may then be applied to an audio amplifier 120 and thereafter output as sound from a speaker 122.
A problem arises because the output of the oscillator 110 generates harmonics that can interfere with certain received input signals 100. These harmonic frequencies typically are coupled to and amplified by the low noise amplifier 104, and can mask the received signal 100. One known technique for reducing such co-channel interference is to shield the oscillator 110, but such shielding may not achieve the desired sensitivity required for a system.
Accordingly, the inventors have determined that there is a need for an RF receiver with enhanced sensitivity and reduced co-channel interference. The present invention meets this need.